EP2884880B1 - Method for measuring a dental object - Google Patents

Description

technical field

The invention relates to a method for measuring a dental object using a dental camera, several optical three-dimensional images of the object being generated during the measurement.

State of the art

A number of methods and dental cameras for optically measuring a dental object are known from the prior art.

In DE 10 2006 009 177 A1 a dental handpiece, in particular a camera handpiece, is disclosed. This document also shows a method according to the preamble of claim 1. When a piezo film is actuated, a recording is made. A recording can be triggered by an acoustic signal by means of an acoustic display device. More precise details on the configuration of the acoustic display device and on the type of acoustic signal are not disclosed.

In addition revealed DE 10 2008 024 817 A1 a camera handpiece that generates a tactile alert for the user when a recording is triggered. The tactile alarm is generated by a tactile display device that includes a piezoelectric oscillator or a magnetic oscillator visual display device such as by means of a monitor.

A disadvantage of the dental cameras and the methods mentioned is that the user has to keep looking at a monitor while the dental object is being measured in order to obtain feedback on the current status or on the recording conditions. This can lead to a longer survey time.

DE102008054985A1 discloses a method and a device for the optical measurement of three-dimensional objects by means of a hand-held, dental 3D camera using a triangulation method. This document also shows a method according to the preamble of claim 1.

DE102007005625A1 discloses a dental camera for 3D measurement of intraoral objects with optical means. EP1434028A1 discloses an apparatus and method for generating a combined image from sub-images.

The object of the present invention is therefore to provide a method for measuring that allows simple and reliable monitoring of the measuring process.

Presentation of the invention

This object is solved by the method in claim 1.

Embodiments of the invention relate to a method for measuring a dental object using a dental camera, with multiple optical three-dimensional images of the object being generated during the measurement. During the measurement, an acoustic sound is generated using a sound generator. The sound serves as a feedback for the user and provides the user with information about a current status of registration of the recordings and/or about certain recording conditions of the dental camera. During the optical measurement, several three-dimensional optical images of the dental object are recorded and then combined to form an overall image. After each individual recording, a computer is used to automatically check whether an overlapping area between the recordings to be joined satisfies certain registration conditions for error-free registration. The sound is generated by the sound generator if the registration conditions for the checked recording are met. Alternatively, the sound can be generated if the registration conditions for the reviewed recording are no longer met. The registration conditions are a sufficient size of the overlapping area, a sufficient waviness of the object surface in the overlapping area, a sufficient roughness of the surface in the overlapping area, a sufficient number of characteristic geometries in the overlapping area, a sufficient number of measuring points in the overlapping area and/or a sufficient recording quality of the recording in the overlap area.

For example, the ICP registration method (Iterative Closest Point Algorithm) can be used as the registration method. This algorithm is a known method for registering two-dimensional or three-dimensional objects. The aim of this method is to map two different 3D models of an object to each other with approximately the same accuracy. To do this, different rotations and translations are applied to corresponding pairs of points in the two 3D models. In doing so, a quadratic error of the distances between the pairs of points is minimized. In detail, in the first step, the nearest neighbors of a given point determined. In the next step, the transformation for registration is calculated. The calculated transformation is then applied to the pairs of points to be registered. This iterative approximation is carried out until the two 3D models match in the overlapping area.

As an alternative or in addition to this, the registration can also be based on the color of the recorded object, the surface curvature of the recorded object or on the basis of geometric matches. Registration based on geometric matches uses a pattern recognition algorithm in which the second image is searched for a specific geometric pattern, such as an occlusal surface of a specific tooth, from the first image.

The registration conditions are not met, for example, if the dental camera is moved too quickly in relation to the object and the size of the overlapping area is therefore not sufficient. Another reason could be that an autofocus of the dental camera is not sharp and the object is therefore not clearly imaged, so that the recording quality of the respective recording is not sufficient. Another reason could be that moving objects such as the patient's tongue or the dentist's fingers are recorded during the measurement. This results in the overlapping areas of the recording not matching. Another reason for incorrect registration could be that the optics of the dental camera fog up due to the high humidity in a patient's oral cavity, which degrades the recording quality. The reasons mentioned could therefore lead to the previously specified registration conditions are not met. In such a case, the sound can then be generated as a beep to alert the user to an incorrect registration. Alternatively, a specific sound can also be generated with each successful registration. In a further alternative, a first sound can be generated in the event of a successful registration and a second sound that differs significantly from the first sound in the event of an incorrect registration.

This prevents erroneous registration between the shots to be joined. Due to the sufficient brightness and roughness of the surface of the overlapping area, reliable registration is made possible in contrast to a flat surface. The sufficient number and arrangement of the characteristic geometries, such as fissures or tooth cusps, also enables reliable registration. If the recording quality is sufficient, the dental object is imaged sharply and with high contrast. A reason for low contrast could be, for example, insufficient lighting of the object. One reason for the blurry picture could be, for example, an incorrectly set autofocus. A sufficient number of measuring points in the overlap area is also required for a successful mission. So only when the number of measuring points in the transition area falls below a specified limit value is the sound generated as a signal tone for faulty registration.

The optical recordings are measured using the dental camera, which can function according to a fringe projection method, for example. With a fringe projection method a pattern of several parallel strips is projected onto the object to be measured and a three-dimensional image of the object is generated on the basis of the distortion of these strips using a triangulation method.

Alternatively, the optical measurement can be carried out using a dental camera, which is based on a confocal optical method or on a color stripe projection method.

In the color stripe projection method, a pattern of multiple color stripes is projected onto the object. Then the depth coordinates for the measuring points are determined and a 3D model of the object is generated. The colored stripes can be clearly identified by their color. For example, four color strips or three color transitions can be used for the color coding of the color strips. The color strips can be generated, for example, using a slide.

The stripe width for such stripe projection methods can be, for example, 130 μm in the measurement volume on the object to be measured.

Another stripe projection method can also be used for the optical measurement, in which the strips are encoded using different light properties, such as intensity, color, polarization, coherence, phase, contrast, location or transit time.

A so-called confocal chromatic triangulation method can also be used for the measurement, in which the concepts of a confocal measurement and a triangulation method are combined with one another. The basic idea is that the surface of a object is colored in such a way that a height coordinate can be deduced directly from a color. The colors are generated by spectral splitting of the projected light, with each wavelength being focused on its own height coordinate.

During the measurement, the hand-held dental camera is moved relative to the dental object, such as a lower jaw or an upper jaw, with the three-dimensional optical recordings being generated at regular time intervals. The individual recordings can be generated, for example, with a clock frequency between 10 Hz and 20 Hz. The individual recordings can then be registered using a computer and combined to form an overall recording.

The sound generator can be a loudspeaker that is integrated into the dental camera. The acoustic sound generated can be designed as desired in terms of its duration, its pitch and/or its volume. The sound can be designed, for example, as a signal tone for exceeding specified recording conditions, such as for too poor contrast or for insufficient sharpness of the respective recording. The sound can also be a beep for incorrect registration of the recordings to be joined.

An advantage of this method is that the user receives acoustic information about the current status of the registration and/or about certain recording conditions of the dental camera directly via the generated sound. As a result, it is no longer necessary for the user, like a dentist, to look at a monitor several times during the measurement in order to determine certain values monitor admission conditions or the status of registration.

An object distance of the object to be measured relative to the dental camera can advantageously be determined, with the sound being generated as a function of this object distance.

The object distance can be determined using the three-dimensional recordings. When creating the three-dimensional images, several measuring points are recorded on the object surface and the 3D coordinates are calculated. This means that the distance between the camera and the respective measuring points is also known. When determining the object distance, the distances of the respective measuring points within a defined object section are averaged. For example, the distances can only be averaged for those measurement points that lie on a defined occlusal surface of a specific tooth.

As a further development, an occlusal surface can also be subdivided into a number of sectors, with a focal point being formed for each sector and only the distances between the camera and the respective focal points of the sectors being averaged. The object distance can therefore be calculated directly for each individual three-dimensional image.

As an alternative to this, the distance measuring unit can measure the distance between the dental camera and the object, for example using an ultrasonic time-of-flight method, also known as a sonar method. In this case, an ultrasound is generated by the distance measuring unit, which is reflected by the object and is measured by a sensor in the distance measuring unit. The distance to the object can then be calculated based on the measured transit time of the ultrasound will. The advantage of this method is that the distance can be measured very quickly without the need for test focusing or contrast measurement.

A focal distance between a focal point of an object surface of the object to be measured and a focal plane of the dental camera can advantageously be determined on the basis of the object distance, with the sound being generated as a function of this focal distance.

With the fringe projection methods and confocal methods used, the emitted light is focused on a specific focal plane or on a sharp layer that is defined by the settings of the focusing optics in the dental camera. The distance between the focal plane and the dental camera is therefore known and can be calculated using a pinhole camera model. Therefore, the focal distance between the focal plane and the centroid of the object surface can be calculated by subtracting the object distance from the focal plane distance relative to the dental camera.

The distance to the focal plane can vary slightly within a measuring field, so that the average distance to the focal plane within the measuring field can be used as a relevant measurement variable for sound generation.

Advantageously, the generated sound can be a modulated sound and its pitch and/or its volume can be changed depending on the focal distance between the object surface and the focal plane of the dental camera.

For example, the modulated sound could be a dissonant tone that indicates focus distance via pitch and/or volume.

Advantageously, the sound may be a short click sound that is generated as soon as specified limits of an in-focus area of the dental camera are exceeded.

For example, the limits of an in-focus area may be +5mm and -5mm relative to the focal plane.

A further method of the invention relates to a method for measuring a dental object using a dental camera, with several optical three-dimensional recordings of the object being generated during the measurement. During the measurement, an acoustic sound is generated using a sound generator. The sound serves as feedback for the user and conveys information to the user about a current status of registration of the recordings and/or about specific recording conditions of the dental camera. A movement speed of the dental camera relative to the object is determined using a movement sensor on the dental camera or by evaluating the measured recordings, with the sound being generated as a function of the movement speed determined.

The optical recordings are measured using the dental camera, which can function according to a fringe projection method, for example. In a stripe projection method, a pattern of several parallel strips is projected onto the object to be measured and a three-dimensional recording of the object is generated using a triangulation process on the basis of the distortion of these strips.

Alternatively, the optical measurement can be carried out using a dental camera that is based on a confocal optical method or based on a color stripe projection method.

In the color stripe projection method, a pattern of multiple color stripes is projected onto the object. Then the depth coordinates for the measuring points are determined and a 3D model of the object is generated. The colored stripes can be clearly identified by their color. For example, four color strips or three color transitions can be used for the color coding of the color strips. The color strips can be generated, for example, using a slide.

The stripe width for such stripe projection methods can be, for example, 130 μm in the measurement volume on the object to be measured.

Another stripe projection method can also be used for the optical measurement, in which the strips are encoded using different light properties, such as intensity, color, polarization, coherence, phase, contrast, location or transit time.

A so-called confocal chromatic triangulation method can also be used for the measurement, in which the concepts of a confocal measurement and a triangulation method are combined with one another. The basic idea is that the surface of an object is colored in such a way that a height coordinate can be inferred directly from a color. The colors are generated by spectral splitting of the projected light, with each wavelength being focused on its own height coordinate.

During the measurement, the hand-held dental camera becomes relative to the dental object, such as a lower jaw or an upper jaw, with the three-dimensional optical recordings being generated at regular time intervals. The individual recordings can be generated, for example, with a clock frequency between 10 Hz and 20 Hz. The individual recordings can then be registered using a computer and combined to form an overall recording.

The sound generator can be a loudspeaker that is integrated into the dental camera. The acoustic sound generated can be designed as desired in terms of its duration, its pitch and/or its volume. The sound can be designed, for example, as a signal tone for exceeding specified recording conditions, such as for too poor contrast or for insufficient sharpness of the respective recording. The sound can also be a beep for incorrect registration of the recordings to be joined.

The movement sensor can be integrated into the dental camera and can determine the relative movement speed of the dental camera relative to the object. However, the relative movement speed can also be determined by evaluating the measured recordings, with the movement speed being determined on the basis of an offset of the object in successive recordings and on the basis of the known time interval between the recordings.

As an alternative to this, the movement speed can be determined by evaluating the measured recordings. In this case, for example, the ICP method can be used. In this case, relative transformations of the measured object are calculated in the successively recorded individual recordings. These transformations can represent translations and rotations of the object. With knowledge of the recording frequency and the determined transformations, the movement speed for individual measuring points of the object can then be determined. An average speed of movement within a measuring field can be used as a measured variable for the sound generation. As a result, the movement speeds of the individual measuring points within the measuring field that occur when the camera rotates are averaged out. The measuring field can have a size of 17 mm×14 mm, for example.

An advantage of this method is that the user receives acoustic information about the current status of the registration and/or about certain recording conditions of the dental camera directly via the generated sound. As a result, it is no longer necessary for the user, like a dentist, to look at a monitor several times during the measurement in order to monitor certain recording conditions or the status of the registration.

The sound generator can advantageously emit a modulated sound, with the pitch and/or the volume of the sound being able to be changed depending on the speed of movement of the dental camera relative to the object.

In this way, the modulated sound indicates the relative speed of movement via pitch and/or volume.

Advantageously, the sound produced can be a short click sound, which is produced as soon as a specified critical speed of movement is exceeded.

As a result, the user is informed by means of such a signal tone that the critical movement speed has been exceeded.

The critical movement speed can be 2 cm/second, for example.

Brief description of the drawings

Fig. 1

eine Skizze zur Verdeutlichung des vorliegenden Verfahrens zur Vermessung eines dentalen Objekts, die

Fig. 2

eine Skizze zur Verdeutlichung einer Klangerzeugung abhängig von einem Objektabstand, die

Fig. 3

eine Skizze zur Verdeutlichung einer alternativen Ausführungsform für die Bestimmung des Objektabstandes, die

Fig. 4

ein Diagramm zur Verdeutlichung der Erzeugung eines modulierten Klangs in Abhängigkeit vom Objektabstand, die

Fig. 5

eine Skizze zur Verdeutlichung der Bestimmung der Bewegungsgeschwindigkeit anhand der Aufnahmen.

The invention is explained with reference to the drawings. It shows that

1

a sketch to illustrate the present method for measuring a dental object, the

2

a sketch to clarify a sound generation depending on an object distance, the

3

a sketch to illustrate an alternative embodiment for determining the object distance, the

4

a diagram to clarify the generation of a modulated sound depending on the object distance, the

figure 5

a sketch to clarify the determination of the movement speed based on the recordings.

example

the 1 shows a sketch to clarify the present method for measuring a dental object 1, that of a lower jaw, by means of a dental camera 2, with several individual optical three-dimensional recordings 3 being generated during the measurement. The three-dimensional images 3, in the form of rectangles are shown are measured using the dental camera 2, which is moved along a measurement path 4 relative to the object 1. The dental camera 2 can be, for example, a hand-held camera that measures the object 1 using a fringe projection method. The strips can be encoded using different light properties, such as intensity, color, polarization, coherence, phase, contrast, location and propagation time. A confocal measurement method can also be used as the measurement method.

The individual recordings 3 are combined into an overall recording 5 by means of a registration method, with a computer 6 automatically checking whether the overlapping areas 7, which are shown in dashed lines, meet certain registration conditions for error-free registration. The registration conditions can be a sufficient size of the overlapping areas 7, a sufficient waviness of the object surface overlapping area, a sufficient roughness of the surface overlapping area, a sufficient number of characteristic geometries in the overlapping area 7, a sufficient number of measuring points in the overlapping area 7 and/or a sufficient recording quality of the recording 3 in the Overlap area 7 be. The third overlapping area 8 does not meet these registration conditions since the size of this overlapping area 8 is too small for successful registration. In such a case, a characteristic sound 10, which is represented by the radial lines, is generated as feedback for the user by means of a sound generator 9, which is arranged in the dental camera 2. Alternatively, a second sound may be generated if the registration conditions are met. As an alternative or in addition to In addition to the first sound generator 9, a second sound generator 11 can be arranged outside the dental camera 2, for example inside the computer 6, which generates a second characteristic sound 12. The sounds 10, 12 can be configured in any way. The sound can be, for example, a characteristic click sound or a sound whose pitch or volume is modulated. During the measurement, further individual recordings 3 can be registered for the overall recording 5 until the entire dental object 1 has been measured. The overall recording 5 can be displayed, for example, by means of a display device 13, such as a monitor, with input means 14, such as a mouse and a keyboard, being able to be connected to the computer 6. The user can then use the input means 14 via a cursor 15 to rotate and move the three-dimensional overall recording 5 in order to change the viewing direction.

As an alternative to this, the sound can be generated as a function of an object distance 16 between the dental camera 2 and the object 1 to be measured.

In a further alternative, the sound can be generated as a function of a movement speed of the dental camera 2 along the measurement path 4 relative to the object 1.

the 2 shows a sketch to illustrate a characteristic sound 10 by means of the sound generator 9, which is integrated into the dental camera 2, depending on the object distance 16. In the case shown, the object distance 16 is determined by measuring the individual distances between the measuring points 20 on a fixed occlusal surface 21 specific molars 22 are averaged relative to the dental camera 2 and a center of gravity 23 is thereby calculated which is represented by a capital x. The object distance 16 is therefore the averaged distance between the dental camera 2 and the relevant object surface, such as an occlusal surface 21. The object distance can also be calculated by averaging over all measurement points within a measurement field. A focal plane 24 has a known distance 25 from the dental camera 2, and this distance can be calculated using the pinhole camera model, taking into account the optics used. With a fringe projection method, the fringe produced is sharply imaged in the focal plane. Knowing the determined object distance 16 and the known distance 25 to the focal plane, a focal distance 26 can be calculated by subtraction. If this focal distance 26 exceeds a specified lower limit 27 or an upper specified limit 28, the sound 10 is generated as a signal tone. The specified limits can be +/- 5 mm, for example. The generated sound 10 can also be a modulated sound, the pitch and/or volume of which is modulated as a function of the focal distance 26 .

the 3 shows a sketch to clarify an alternative embodiment for determining the object distance 16 from 2 . The relevant occlusal surface 21 is divided into four sectors 30, 31, 32 and 33, with a first focal point 34, a second focal point 35, a third focal point 36 and a fourth focal point 37 being determined for each of the sectors. The distances between the respective focal points 34, 35, 36 and 37 relative to the camera 2 are then averaged and a focal point of the entire occlusal surface 21 is thereby determined.

the 4 shows a sketch to clarify the generation of a modulated sound, the graph shown representing the pitch or the frequency of the generated sound 40 as a function of the object distance 16 . The pitch of the sound is lowest in the focal plane 24 and rises slowly up to the defined limits 27 and 28 . When the limits 27 or 28 are crossed, the pitch 40 increases suddenly in the areas 41 and 42, so that the dentist can clearly see that the defined limits 27 or 28 have been exceeded.

the figure 5 shows a sketch to clarify the determination of the movement speed of the dental camera 2 relative to the object 1 1 using an ICP algorithm. A section of the object 1 is recorded in a first recording 50 by the dental camera 2 along the measurement path 4 1 moved and a subsequent second recording 51 performed, wherein the section of the object 1 was shifted. A translation 52, which can be a combination of a linear displacement and a rotation, is determined using the ICP algorithm. Then, knowing the recording frequency and the determined translation 52, a movement speed is determined for each of the measuring points on the object 1. An average movement speed is then calculated by averaging the movement speed of the individual measurement points. As soon as a critical movement speed is exceeded, the sound can turn off 10 1 are generated to indicate to the dentist that he is moving the dental camera 2 too quickly.

Reference sign

1

object

2

camera

3

recording

4

survey way

5

overall recording

6

computer

7

overlap area

8th

overlap area

9

sound generator

10

sound

11

sound generator

12

sound

13

overall recording

14

input means

15

cursor

16

object distance

20

measuring point

21

occlusal surface

22

molar tooth

23

main emphasis

24

focal plane

25

Distance

26

focus distance

27

Border

28

Border

30

sector

31

sector

32

sector

33

sector

34

main emphasis

35

main emphasis

36

main emphasis

37

main emphasis

40

pitch

41

area

42

area

50

recording

51

recording

52

translation

Claims (1)

1. Method for measuring a dental object (1) using a dental camera (2), wherein several optical three-dimensional images (3, 50, 51) of the object (1) are produced during the measurement, wherein an acoustic sound (10, 12) is generated by a sound generator (9, 11) during the measurement, characterised in that the sound (10, 12) is suitable as feedback for the user, wherein the speed of movement of the dental camera (2) relative to the object (1) is determined using a motion sensor on the dental camera (2) or by evaluating the measured images (50, 51), wherein the sound (10, 12) is generated depending on the determined speed of movement.

Images